Two-degree-of-freedom, dry tuned rotor gyroscopes are well known. In order to be put into practical use, an electrical control system, known as a "rebalance loop", must be provided in order to control the gyroscope over its dynamic range and provide a precise output measurement of angular rate of the gyroscope rotor about each of the rotor axes. The dynamic range of such an instrument may vary from 0.01 degree/hour to greater than 1 radian/second requiring electrical rebalance currents ranging from several microamperes to several hundred milliamperes. In addition, the dry tuned rotor gyroscope is, by nature, an instrument which is susceptible or sensitive to angular acceleration. A rebalance loop must compensate for angular acceleration if full gyroscope performance is to be realized. In typical dry tuned rotor gyroscopes, flexures are used to support the rotor. Flexures are small metallic blades supporting the rotor and arranged between the motor shaft and the rotor in such a was as to give the rotor freedom of angular motion in two orthogonal axes. In a strapdown arrangement, the gyros are secured to the body axes of the vehicle and are required to sense the full range of dynamics of the vehicle. The strapdown arrangement greatly reduces the mechanical complexity and cost of the gyroscope and therefore of the system, such as an inertial navigation system for example, with which it is used, but increases the electronic complexity with respect to the rebalance electronics. The gyros are provided with a "pickoff" for each degree of freedom or axis of the gyro for producing an electrical signal in response to angular motion applied to that axis and a "torquer" associated with each such axis for applying a compensating torque about that axis. The pickoff signals are amplified and delivered to the rebalance loop which processes the signal and produces a torquer current which is delivered to the torquer. This causes the torquer to apply a torque to the rotor so as to maintain the rotor in a "null" position. Rebalance loops typically include a direct-axis loops or circuits which receive the signal from the pickoff associated with one axis and delivers a torquer current to the torquer associated with the other axis.
Several types of strapdown rabalance loops are known including pulsed loops, of either the binary or pulse width modulated types, and analog loops. For lower performance instruments and systems, any of these types of rebalance loops can be used if suitably modified to suit the specific gyroscope of interest. High performance, strapdown, inertial gyroscopes having a wide dynamic range and effects due to acceleration require modified rebalance loop designs. Most, if not all, analog rebalance loops use the same principle of operation to generate rate information from the torquing current and that is by allowing the torquing current to flow through a precision resistor to obtain a voltage signal that is proportional to the torque. Usually, the precision resistor is in the "return" lead of the torquer coils so that one node of that resistor can be referenced to ground. A rebalance loop is a type of servo control that is inherently unstable unless the loop contains phase compensation or some form of control scheme. The most simple scheme of phase compensation is a phase lag in the direct-axis loop and is used in analog as well as digital rebalance loops. Another scheme of which the inventors are aware, although it is not known whether it is used in rebalance loops of the type with which the present invention is considered, is the so-called "cross-coupled loop" which uses cross-axis circuits for the purpose of making the loop stable. The cross-coupled loop uses differentiators as cross-axis circuits only to stabilize nutation but do not provide compensation for angular acceleration.